THE ELEVTItIV LIGHT. 677 



furnishes a direct current, which is carried round the 

 electro- magnets of a second and larger machine. Wilde's 

 principle, it may be added, is also applied on the Thames 

 Embankment and the Holborn Viaduct; a small Gramme 

 machine being used in each case to excite the electro- 

 magnets of the large one. 



The Farmer- Wallace machine is also an apparatus of 

 great power. It consists of a combination of bobbins for 

 induced currents, and of inducing electro magnets, the 

 latter being excited by the method discovered by Siemens 

 and Wheatstone. In the machines intended for the 

 production of the 'electric light, the electro-motive force is 

 so great as to permit of the introduction of several lights 

 in the same circuit. A peculiarly novel feature of the 

 Farmer- Wallace system is the shape of the carbons. In- 

 stead of rods, two large plates of carbons with beveled 

 edges are employed, one above the other. The electric 

 discharge passes from edge to edge, and shifts its position 

 according as the carbon is dissipated. The duration of the 

 light in this case far exceeds that obtainable with rods. I 

 have myself seen four of these lights in the same circuit 

 in Mr. Ladd's workshop in the City, and they are 

 now, I believe, employed at the Liverpool Street Station 

 of the Metropolitan Railway. The Farmer-Wallace 

 " quantity machine " pours forth a flood of electricity of 

 low tension. It is unable to cross the interval necessary 

 for the production of the electric light, but it can fuse 

 thick copper wires. When sent through a short bar of 

 iridium, this refractory metal emits a light of extraordinary 

 splendor.* 



The machine of M. de Meritens, which he has gener- 

 ously brought over from Paris for our instruction, is the 

 newest of all. In its construction he falls back upon the 

 principle of the magneto-electric machine, employing 

 permanent magnets as the exciters of the induced currents. 

 Using the magnets of the Alliance Company, by a skillful 

 disposition of his bobbins. M. de Meritens produces with 

 eight magnets a light equal to that produced by forty mag- 

 nets in the Alliance machines. While the space occupied 

 is only one-fifth, the cost is little more than one-fourth 



*The iridium light was shown by Mr. Ladd. It brilliantly illu- 

 minated the theater of the Royal Institution. 



